Grain-oriented electrical steel sheet and method for manufacturing same

ABSTRACT

The present invention relates to a grain-oriented electrical steel sheet and a method for manufacturing the same. The grain-oriented electrical steel sheet contains 2.0 wt % or more to 5.0 wt % or less of Si, 0.005 wt % or more to 0.04 wt % or less of acid-soluble Al, 0.01 wt % or more to 0.2 wt % or less of Mn, 0.01 wt % or less (excluding 0 wt %) of N, 0.01 wt % or less (excluding 0 wt %) of S, 0.01 wt % or more to 0.05 wt % or less of Sb, 0.02 wt % or more to 0.08 wt % or less of C, 0.0005 wt % or more to 0.045 wt % or less of P, 0.03 wt % or more to less than 0.08 wt % of Sn, and 0.01 wt % or more to 0.2 wt % or less of Cr, and contains balance Fe and other inevitable impurities.

TECHNICAL FIELD

The present invention relates to a grain-oriented electrical steel sheet and a method for manufacturing the same.

BACKGROUND ART

A grain-oriented electrical steel sheet is a soft magnetic material having excellent magnetic characteristics in one direction or a rolling direction since a texture of a bloom with respect to the rolling direction is a Goss texture, which is {110}<001>. In order to reveal such a texture, complicated processes such as component control in steelmaking, slab reheating and hot rolling process factor control in hot rolling, hot-rolled sheet annealing heat treatment, primary recrystallization annealing, and secondary recrystallization annealing, and the like, are required, and need to be very precisely and strictly managed. Meanwhile, it is also very important to control inhibitors, which are one of factors revealing the Goss texture, that is, crystal grain growth inhibitors inhibiting indiscriminate growth of primary recrystallized grains and allowing only the Goss texture to be grown at the time of generation of the secondary recrystallization. In order to obtain the Goss texture in final annealing, growth of all the primary recrystallized grains needs to be inhibited until just before the secondary recrystallization is generated, and in order to obtain sufficient inhibition ability for the inhibition of the growth, an amount of inhibitors needs to be sufficiently large and a distribution of the inhibitors needs to be uniform. Meanwhile, in order to allow the secondary recrystallization to be generated during a high-temperature final annealing process, the inhibitors needs to have excellent thermal stability so as not to be easily decomposed. The secondary recrystallization is a phenomenon occurring since the inhibitors inhibiting the growth of the primary recrystallized grains are decomposed in an appropriate temperature section and lose the inhibition ability, at the time of the final annealing. In this case, specific crystal grains such as Goss crystal grains are rapidly grown in a relatively short time.

Generally, quality of a grain-oriented electrical steel sheet may be evaluated by a magnetic flux density and core loss, which are typical magnetic characteristics, and the higher the precision of the Goss texture, the more excellent the magnetic characteristics. In addition, a grain-oriented electrical steel sheet having excellent quality may manufacture an electric power device having high efficiency due to material characteristics, such that miniaturization and efficiency improvement of the electric power device may be accomplished.

The related arts overcome a limitation of cold rolling through warm rolling after increasing a content of silicon or decrease the core loss by increasing a specific resistance through siliconizing, in order to improve the magnetic characteristics of the grain-oriented electrical steel sheet. However, in this case, an additional process is required, and a manufacturing cost is increased. In addition, schematic configurations of technologies of manufacturing a grain-oriented electrical steel sheet by adding alloy elements such as Ti, B, Se, Sb, Sn, Cr, and the like, in order to improve crystal grain growth inhibition ability are described. However, ranges of the alloy elements are generally described excessively widely, a description for an effect of each of the alloy elements is small, and a case in which the grain-oriented electrical steel sheet includes two or more kinds or alloy elements rather an effect of a single alloy element are mainly described. That is, according to the known technologies up to now, it is described only that the magnetic characteristics of the grain-oriented electrical steel sheet may be improved by adding Ti, B, Se, Sb, Sn, Cr, and the like, and direct effects, appropriate contents, and identification of a synergy effect by an interaction between two or more kinds or alloy elements when the two or more alloying elements are added are hardly described. That is, a detailed method capable of appropriately exerting effects of the alloy elements described above is not provided, and even though the detailed method capable of appropriately exerting effects of the alloy elements described above is provided, causes and relationship identification are not enough.

DISCLOSURE Technical Problem

The present invention has been made in an effort to provide a grain-oriented electrical steel sheet and a method for manufacturing the same having advantages of having small core loss and an excellent magnetic flux density.

Technical Solution

An exemplary embodiment of the present invention provides a grain-oriented electrical steel sheet including 2.0 wt % or more to 5.0 wt % or less of Si, 0.005 wt % or more to 0.04 wt % or less of acid-soluble Al, 0.01 wt % or more to 0.2 wt % or less of Mn, 0.01 wt % or less (excluding 0 wt %) of N, 0.01 wt % or less (excluding 0 wt %) of S, 0.01 wt % or more to 0.05 wt % or less of Sb, 0.02 wt % or more to 0.08 wt % or less of C, 0.0005 wt % or more to 0.045 wt % or less of P, 0.03 wt % or more to less than 0.08 wt % of Sn, and 0.01 wt % or more to 0.2 wt % or less of Cr, and including balance Fe and other inevitable impurities.

The grain-oriented electrical steel sheet may satisfy the following Equation 1 calculated by contents (wt %) of the respective components:

−0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si.  [Equation 1]

The grain-oriented electrical steel sheet may satisfy the following Equation 2 calculated by contents (wt %) of the respective components:

Sn+Sb≤5Cr.  [Equation 2]

The grain-oriented electrical steel sheet may satisfy the following Equation 1 and the following Equation 2 calculated by contents (wt %) of the respective components:

−0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si, and  [Equation 1]

Sn+Sb≤5Cr.  [Equation 2]

A fracture of an austenite phase in the grain-oriented electrical steel may be 20 to 30%.

An area of crystal grains of which a ratio between the longest diameters and the shortest diameters is 1.0 or more among crystal grains of which lengths of the shortest diameters are 3 mm or more may be 5% or more of an area of all the crystal grains.

Another exemplary embodiment of the present invention provides a method for manufacturing a grain-oriented electrical steel sheet including reheating a steel slab containing 2.0 wt % or more to 5.0 wt % or less of Si, 0.005 wt % or more to 0.04 wt % or less of acid-soluble Al, 0.01 wt % or more to 0.2 wt % or less of Mn, 0.01 wt % or less (excluding 0 wt %) of N, 0.01 wt % or less (excluding 0 wt %) of S, 0.01 wt % or more to 0.05 wt % or less of Sb, 0.02 wt % or more to 0.08 wt % or less of C, 0.0005 wt % or more to 0.045 wt % or less of P, 0.03 wt % or more to less than 0.08 wt % of Sn, and 0.01 wt % or more to 0.2 wt % or less of Cr, and including balance Fe and other inevitable impurities; manufacturing a steel sheet by performing hot rolling, hot-rolled sheet annealing, and cold rolling on the reheated steel slab; performing decarbonization annealing and nitriding annealing on the cold-rolled steel sheet; and finally annealing the decarbonization annealed and nitriding annealed steel sheet.

The steel slab may satisfy the following Equation 1 calculated by contents (wt %) of the respective components:

−0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si.  [Equation 1]

The steel slab may satisfy the following Equation 2 calculated by contents (wt %) of the respective components:

Sn+Sb≤5Cr.  [Equation 2]

The steel slab may satisfy the following Equation 1 and the following Equation 2 calculated by contents (wt %) of the respective components:

−0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si, and  [Equation 1]

Sn+Sb≤5Cr.  [Equation 2]

In the reheating of the steel slab, a temperature may be 1000 to 1250° C.

In the manufacturing of the steel sheet by performing the hot rolling, the hot-rolled sheet annealing, and the cold rolling on the reheated steel slab, a hot-rolled sheet annealing temperature may be 900 to 1200° C.

In the manufacturing of the steel sheet by performing the hot rolling, the hot-rolled sheet annealing, and the cold rolling on the reheated steel slab, a cold rolling thickness may be 0.10 mm or more to 0.50 mm or less.

In the manufacturing of the steel sheet by performing the hot rolling, the hot-rolled sheet annealing, and the cold rolling on the reheated steel slab, the cold rolling may be performed as once cold rolling of which a cold rolling ratio is 87% or more.

In the performing of the decarbonization annealing and the nitriding annealing on the cold-rolled steel sheet, the decarbonization annealing and the nitriding annealing may be simultaneously performed, the nitriding annealing may be independently performed after the decarbonization annealing, or the decarbonization annealing may be independently performed after the nitriding annealing.

In the performing of the decarbonization annealing and the nitriding annealing on the cold-rolled steel sheet, the decarbonization annealing and the nitriding annealing may be simultaneously performed, and an annealing temperature may be 800 to 950° C.

The method for manufacturing a grain-oriented electrical steel sheet may further include applying an annealing separating agent to the decarbonization annealed and nitriding annealed steel sheet.

In the final annealing of the decarbonization annealed and nitriding annealed steel sheet, a final annealing temperature may be 800 to 1250° C.

The final annealing of the decarbonization annealed and nitriding annealed steel sheet may be performed under an atmosphere including one or more of nitrogen and hydrogen, and may be performed under a 100% hydrogen atmosphere after a temperature arrives at the final annealing temperature.

Advantageous Effects

A grain-oriented electrical steel sheet having small core loss and an excellent magnetic flux density, and a method for manufacturing the same are provided.

DESCRIPTION OF THE DRAWINGS

Hereinafter, an exemplary embodiment of the present invention is described in detail. However, it is to be understood that this exemplary embodiment is provided as an example, and the present invention is not limited by this exemplary embodiment, but is defined by only the scope of claims to be described below.

In the present specification, “%” means wt % unless defined otherwise.

An exemplary embodiment of the present invention provides a grain-oriented electrical steel sheet containing 2.0 wt % or more to 5.0 wt % or less of Si, 0.005 wt % or more to 0.04 wt % or less of acid-soluble Al, 0.01 wt % or more to 0.2 wt % or less of Mn, 0.01 wt % or less (excluding 0 wt %) of N, 0.01 wt % or less (excluding 0 wt %) of S, 0.01 wt % or more to 0.05 wt % or less of Sb, 0.02 wt % or more to 0.08 wt % or less of C, 0.0005 wt % or more to 0.045 wt % or less of P, 0.03 wt % or more to less than 0.08 wt % of Sn, and 0.01 wt % or more to 0.2 wt % or less of Cr, and containing balance Fe and other inevitable impurities.

In detail, the grain-oriented electrical steel sheet may satisfy the following Equation 1 calculated by contents (wt %) of the respective components:

−0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si.  [Equation 1]

In detail, the grain-oriented electrical steel sheet may satisfy the following Equation 2 calculated by contents (wt %) of the respective components;

Sn+Sb≤5Cr.  [Equation 2]

In more detail, the grain-oriented electrical steel sheet may satisfy the following Equation 1 and the following Equation 2 calculated by contents (wt %) of the respective components:

−0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si, and  [Equation 1]

Sn+Sb≤5Cr.  [Equation 2]

Contents (wt %) of Mn, Si, and C are controlled to satisfy the above Equation 1. This is to allowing a phase transformation fracture of an austenite phase that cannot but be inevitably generated in temperature ranges of steel slab reheating, hot rolling, and the subsequent hot-rolled sheet annealing at the time of manufacturing the grain-oriented electrical steel sheet to be maintained in 20 to 30%. In the case in which an amount of austenite phase is excessively small, a hot-rolled sheet microstructure may remain in a final product after high-temperature annealing to deteriorate the magnetic characteristics. In the case in which an amount of austenite phase is excessively large, α+γ phase transformation may be excessively activated during primary recrystallization annealing (decarbonization annealing), resulting in damage to a Goss texture.

The contents (wt %) of Sn, Sb, and Cr are controlled to satisfy the above Equation 2. This is to allow an oxide layer of a decarbonized annealed sheet that becomes a base of a base coating to be appropriately formed.

The grain-oriented electrical steel sheet may be a grain-oriented electrical steel sheet in which an area of crystal grains of which a ratio between the longest diameters and the shortest diameters is 1.0 or more among crystal grains of which lengths of the shortest diameters are 3 mm or more is 5% or more of an area of all the crystal grains. In a case of a grain-oriented electrical steel sheet having a large number of crystal grains grown in a rolling direction, magnetic characteristics in the rolling direction demanded by the grain-oriented electrical steel sheet itself may be more excellent.

Hereinafter, the reason why contents of the respective components are limited is described in detail. % means wt %.

Si: 2.0% or more to 5.0% or less

Si, which is a basic composition of the grain-oriented electrical steel sheet, serves to increase a specific resistance of a material to decrease core loss. In the case in which a content of Si is less than 2.0%, the specific resistance is decreased, such eddy current loss is increased, and core loss characteristics are thus deteriorated. In addition, at the time of decarbonization nitriding annealing, phase transformation between ferrite and austenite becomes active, such that a primary recrystallization texture is severely damaged. In addition, at the time of high-temperature annealing, the phase transformation between ferrite and austenite is generated, such that secondary recrystallization becomes unstable, and a {110} Goss texture is severely damaged.

In the case in which the content of Si exceeds 5.0%, at the time of the decarbonization nitriding annealing, SiO₂ and Fe₂SiO₄ oxide layers are excessively and densely formed to delay decarbonization behavior. Therefore, phase transformation between ferrite and austenite is continuously generated during the decarbonization nitriding annealing, such that a primary recrystallization texture is severely damaged. Nitriding behavior is delayed due to a decarbonization behavior delay effect depending on the formation of the dense oxide layer described above, such that nitrides such as (Al,Si,Mn)N, AlN, and the like, are not sufficiently formed. Therefore, sufficient crystal grain inhibition ability required for the secondary recrystallization at the time of the high-temperature annealing may not be secured. In addition, when the content of Si exceeds 5.0%, a brittleness and a roughness, which are mechanical characteristics of the grain-oriented electrical steel sheet, are increased and decreased, respectively, resulting an increase in a sheet fracture occurrence rate in a rolling process. Therefore, weldability between sheets is deteriorated, such that easy workability may not be secured.

Resultantly, when the content of Si is not controlled in the predetermined range described above, formation of the secondary recrystallization becomes unstable. Therefore, magnetic characteristics are severely damaged, and workability is also deteriorated. Therefore, it is preferable that the content of Si is limited to 2.0% or more to 5.0% or less.

acid-soluble Al: 0.005% or more to 0.04% or less

Al forms AlN finely precipitated at the time of hot rolling and hot-rolled sheet annealing or is combined with Al, Si, and Mn in which nitrogen ions introduced by an ammonia gas exist in a solid-dissolved state within steel in an annealing process after cold rolling, thereby forming (Al, Si, Mn) N and AlN type nitrides. The materials described above serve as strong crystal grain growth inhibitors.

In the case in which a content of Al is less than 0.005%, the number and a volume of materials described above are significantly low, such that a sufficient effect of the materials described above as the inhibitors may not be expected.

In the case in which the content of Al exceeds 0.04%, coarse nitrides are formed, such that crystal grain growth inhibition ability is decreased. Therefore, the content of Al is limited to 0.005% or more to 0.04% or less.

Mn: 0.01% or more to 0.2% or less

Mn increases the specific resistance to decrease the eddy current loss, resulting in a decrease in entire core loss, similar to Si. In addition, Mn forms an Mn-based sulfide by reacting to S in a fired steel state, or forms a precipitate of (Al, Si, Mn) N by reacting to nitrogen introduced by the nitriding together with Si. Therefore, Mn is an important element in inhibiting growth of primary recrystallized grains and generating the secondary recrystallization.

In the case in which a content of Mn is less than 0.01%, the number and a volume of materials described above are significantly low, such that a sufficient effect of the materials described above as the inhibitors may not be expected.

In the case in which the content of Mn exceeds 0.2%, large amounts of (Fe, Mn) and Mn oxides are formed in addition to Fe₂SiO₄ on a surface of the steel sheet to hinder the base coating from being formed during the high-temperature annealing, resulting in deterioration of surface quality. In addition, since phase transformation between ferrite and austenite is caused in a high-temperature annealing process, the texture is severely damaged, such that the magnetic characteristics are significantly deteriorated. Therefore, the content of Mn is limited to 0.01% or more to 0.2% or less.

N: 0.01% or less (excluding 0%)

N is an important element forming AlN and BN by reacting to Al and B, and it is preferable to add 0.01% or less of N in a steelmaking step.

When a content of added N exceeds 0.01%, a surface defect such as a blister is caused by nitrogen diffusion in a process after hot rolling. In addition, since an excessive large amount of nitrides are formed in a slab state, rolling becomes difficult, such that the next process may be complicated and a manufacturing cost may be increased. Meanwhile, N additionally required in order to form nitrides such as (Al,Si,Mn)N, AlN, (B,Si,Mn)N, (Al,B)N, BN, and the like, is reinforced by performing nitriding in steel using an ammonia gas in the annealing process after the cold rolling.

C: 0.02% or more to 0.08% or less

C is an element contributing to grain refinement and improvement of an elongation percentage by generating phase transformation between ferrite and austenite. C is an essential element for improving a rolling property of the grain-oriented electrical steel sheet having a poor rolling property due to a high brittleness. However, since C is an element deteriorating the magnetic characteristics by precipitating carbides formed due to a magnetic aging effect in a product sheet in the case in which it remains in a final product, a content of C needs to be appropriately controlled.

When a content of C is less than 0.02% in the range of the content of Si described above, the phase transformation between the ferrite and the austenite is not sufficiently generated, which causes non-uniformity of a slab and a hot rolled microstructure. Therefore, a cold rolling property is damaged.

When the content of C exceeds 0.08% in the range of the content of Si described above, sufficient decarbonization may not be obtained in a decarbonization annealing process unless a separate process or equipment is added. Due to a phase transformation phenomenon caused by the reason described above, a secondary recrystallization texture is severely damaged. Further, a deterioration phenomenon of the magnetic characteristics by magnetic aging is caused at the time of applying a final product to an electric power device.

Therefore, the content of C is limited to 0.02% or more to 0.08% or less.

S: 0.01% or less (excluding 0%)

When a content of S exceeds 0.01%, precipitates of MnS are formed in the slab to inhibit crystal grain growth. In addition, S is segregated at a central portion of the slab at the time of casting, such that it is difficult to control a microstructure in the subsequent process. In addition, since MnS is not used as a crystal grain growth inhibitor in the present invention, it is not preferable that S is added and precipitated by an inevitable content or more. Therefore, it is preferable that the content of S is 0.01% or less.

P: 0.0005% or more to 0.045% or less

P is segregated in grain boundaries to hinder movement of the grain boundaries, and at the same time, may play an auxiliary role of inhibiting crystal grain growth. Therefore, there is an effect of improving a {110}<001> texture in terms of a microstructure. When a content of P is less than 0.0005%, there is no addition effect of P, and when the content of P exceeds 0.045%, a brittleness is increased, such that a rolling property is significantly deteriorated. Therefore, it is preferable that the content of P is limited to 0.0005% or more to 0.045% or less.

Sb: 0.01% or more to 0.05% or less

Sb is segregated in the grain boundaries to hinder crystal grain growth, similar to P, and stabilizes the secondary recrystallization. However, Si has a low melting point, and may thus be easily diffused to the surface during the primary recrystallization annealing to hinder decarbonization, formation of an oxide layer, and nitriding. Therefore, when Sb is added by a predetermined level or more, it hinders the decarbonization and inhibits the formation of the oxide layer that becomes the base of the base coating, and thus, there is an upper limit in a content of added Sb.

As a result of continuous research by the present inventors, it has been found that at least 0.01% of Sb needs to be added in order for a crystal grain growth inhibition effect to appear. In addition, when a content of Sb exceeds 0.05%, the inhibition effect and diffusion of Sb to the surface becomes severs, such that stable secondary recrystallization is not obtained. In addition, it was found that surface quality is deteriorated. Therefore, it is preferable that the content of Sb has a range of 0.01% or more to 0.05% or less.

Sn: 0.03% or more to less than 0.08%

Sn, which is a grain boundary segregated element, similar to P, is an element hindering movement of grain boundaries, and is thus known as a crystal grain growth inhibitor. In a predetermined range of the content of Si of the present invention, crystal grain growth inhibition ability for smooth secondary recrystallization behavior at the time of the high-temperature annealing is insufficient. Therefore, Sn segregated in the grain boundaries to hinder the movement of the grain boundaries is necessarily required.

The present inventors have found that in the case in which a content of Sn is less than 0.03%, an improvement effect of the magnetic characteristics appears as compared with a case in which Si does not exist at all, but is slight, through continuous research. In addition, they have found that in the case in which the content of Sn is 0.08% or more, when a temperature increase speed is not adjusted or maintained for a predetermined time in a primary recrystallization annealing section, crystal grain growth inhibition ability is excessively strong, such that stable secondary recrystallization may not be obtained. Therefore, it is preferable that the content of Sn is 0.03% or more and is less than 0.08.

Cr: 0.01% or more to 0.2% or less

Cr promotes formation of a hard phase in a hot-rolled annealed sheet to promote formation of {110}<001> of the Goss texture at the time of the cold rolling. In addition, Cr promotes decarbonization in a decarbonization annealing process to decrease an austenite phase transformation maintaining time, resulting in prevention of damage to the texture. In addition, Cr promotes formation of an oxide layer of a surface formed in the decarbonization annealing process to complement a disadvantage that formation of the oxide layer is hindered due to Sn and Sb.

The present inventors have found that in the case in which a content of Cr is less than 0.01%, the effect described above appears as compared with a case in which Cr does not exist at all, but is slight, through continuous research. In addition, they have found that in the case in which the content of Cr exceeds 0.2%, the oxide layer is more densely formed in the decarbonization annealing process. Therefore, the formation of the oxide layer is deteriorated, and the decarbonization and the nitriding are hindered. In addition, since C is an expensive alloy element, it is preferable that an upper limit value of the content of Cr is set to 0.2% or less.

The grain-oriented electrical steel sheet according to the present invention described above may be manufactured by a method for manufacturing a grain-oriented electrical steel sheet well-known in the technical field to which the present invention pertains, but it is more preferable that the grain-oriented electrical steel sheet is manufactured by a method for manufacturing a grain-oriented electrical steel sheet to be described below. Hereinafter, a more preferably method for manufacturing a grain-oriented electrical steel sheet is described. Conditions which are not specifically described below are considered to be in accordance with general conditions.

An another exemplary embodiment of the present invention provides a method for manufacturing a grain-oriented electrical steel sheet including reheating a steel slab containing 2.0 wt % or more to 5.0 wt % or less of Si, 0.005 wt % or more to 0.04 wt % or less of acid-soluble Al, 0.01 wt % or more to 0.2 wt % or less of Mn, 0.01 wt % or less (excluding 0 wt %) of N, 0.01 wt % or less (excluding 0 wt %) of S, 0.01 wt % or more to 0.05 wt % or less of Sb, 0.02 wt % or more to 0.08 wt % or less of C, 0.0005 wt % or more to 0.045 wt % or less of P, 0.03 wt % or more to less than 0.08 wt % of Sn, and 0.01 wt % or more to 0.2 wt % or less of Cr, and including balance Fe and other inevitable impurities; manufacturing a steel sheet by performing hot rolling, hot-rolled sheet annealing, and cold rolling on the reheated steel slab; performing decarbonization annealing and nitriding annealing on the cold-rolled steel sheet; and finally annealing the decarbonization annealed and nitriding annealed steel sheet.

In detail, the steel slab may satisfy the following Equation 1 calculated by contents (wt %) of the respective components:

−0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si.  [Equation 1]

In detail, the steel slab may satisfy the following Equation 2 calculated by contents (wt %) of the respective components:

Sn+Sb≤5Cr.  [Equation 2]

In more detail, the steel slab may satisfy the following Equation 1 and the following Equation 2 calculated by contents (wt %) of the respective components:

−0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si, and  [Equation 1]

Sn+Sb≤5Cr.  [Equation 2]

Hereinafter, the method for manufacturing a grain-oriented electrical steel sheet is described in detail.

It is preferable to reheat the steel slab at a temperature range in which precipitates of Al-based nitrides or Mn-based sulfides are incompletely solution-treated or completely solution-treated depending on a chemical equivalent relationship among Al, N, Mn, and S that are solid-dissolved.

In the case in which the precipitates are incompletely solution-treated, sizes of the precipitates are greater than those in the case in which the precipitates are completely solution-treated, even though a precipitation amount is large, once cold rolling is possible.

On the contrary, in the case in which the precipitates are completely solution-treated, after hot-rolled sheet annealing heat treatment, large amounts of nitrides or sulfides are finely formed. Therefore, once cold rolling, which is a subsequent process, may not be easy, but in the case in which a precipitation amount of precipitates is small by the chemical equivalent relationship, the once cold rolling is possible.

It is preferable that contents of N and S again solid-dissolved in fired steel through the reheating of the steel slab are 20 to 50 ppm and 20 to 50 ppm, respectively. The contents of N and S that are again solid-dissolved need to consider contents of Al and Mn contained in the fired steel. The reason is that a nitride and a sulfide used as a crystal grain growth inhibitor are (Al,Si,Mn)N, AlN, and MnS.

A correlation equation in relation to solid solubility of Al and N of 3% of pure silicon steel sheet has been suggested by Iwayama and is as follows.

${{\log \left\lbrack {\% \mspace{11mu} {Al}} \right\rbrack}\left\lbrack {\% \mspace{11mu} N} \right\rbrack} = {{{- 10062}\frac{1}{T(K)}} + 2.72}$

For example, when it is assumed that a content of acid-soluble aluminum is 0.028% and a content of N is 0.0050%, a theoretical solid-dissolving temperature by the equation suggested by Iwayama is 1258° C. In order to heat the slab of the electrical steel sheet as described above, the slab needs to be heated at 1300° C.

However, when the slab is heated to 1280° C. or more, Fayalite corresponding to a compound between silicon having low melting point and iron, which is a base metal, is produced in the steel sheet. Therefore, a surface of the steel sheet melts down, such that hot rolling workability is very difficult, and maintenance is increased by heating due to the melting-down iron.

For the reason described above, that is, the heating, it is preferable to reheat the slab at a temperature of 1250° C. or less in order to enable an appropriate control of the maintenance, cold rolling, and a primary recrystallization texture. In detail, a temperature at which the slab is reheated may be 900° C. to 1250° C., 900° C. to 1200° C., 900° C. to 1150° C., 1000° C. to 1250° C., 1100° C. to 1250° C., 1000° C. to 1250° C., or 1000° C. to 1200° C.

In addition, Iwayama has suggested a correlation equation in relation to solid solubility of Mn and S of 3% of pure silicon steel sheet.

${{\log \left\lbrack {\% \mspace{11mu} {Mn}} \right\rbrack}\left\lbrack {\% \mspace{11mu} S} \right\rbrack} = {{{- 14855}\frac{1}{T(K)}} + 6.82}$

For example, when it is assumed that a content of manganese is 0.04% and a content of S is 0.004%, a theoretical solid-dissolving temperature by the equation suggested by Iwayama is 1126° C. When the slab of the electrical steel sheet as described above is heated at 1150° C., an Mn-based sulfide may be completely solution-treated. In addition, when it is assumed that a content of manganese is 0.06% and a content of S is 0.003%, a theoretical solid-dissolving temperature is 1130° C. Therefore, when the slab of the electrical steel sheet is heated at 1150° C., an Mn-based sulfide may be completely solution-treated.

However, when it is assumed that a content of manganese is 0.1% and a content of S is 0.003%, a theoretical solid-dissolving temperature is 1163° C., and when the slab of the electrical steel sheet is heated at 1150° C., an Mn-based sulfide may not be completely solution-treated, but may be almost completely solution-treated.

A deformed structure stretched in a rolling direction by stress exits in a hot-rolled sheet, and AlN, MnS, or the like, is precipitated during the hot rolling. Therefore, in order to have a uniform recrystallization microstructure and a precipitate distribution of fine AlN before the cold rolling, the hot-rolled sheet needs to be heated up to a heating temperature or less of the slab. It is important to recrystallize the deformed structure through the heating and secure sufficient austenite phases to promote solid dissolution of crystal grain growth inhibitors such as AlN and MnS.

Therefore, it is preferable that a hot-rolled sheet annealing temperature is 900 to 1200° C. in order to maximize a fracture of austenite. In addition, it is preferable to perform crack heat treatment at the temperature range described above and then perform cooling. After the hot-rolled sheet annealing heat treatment to which the heat treatment pattern described above is applied, an average size of precipitates in a strip has a range of 200 to 3000 Å.

After the hot-rolled sheet annealing, cold rolling is performed on the sheet using a reverse rolling mill or a tandem rolling mill so that the sheet has a thickness of 0.10 mm or more to 0.50 mm or less. It is preferable to perform once cold rolling from an initial rolling thickness to a thickness of a final product without performing annealing heat treatment on a deformed structure in an intermediate process.

Orientations of which a degree of integration of a {110}<001> orientation is low are rotated to deformed orientations. Therefore, only Goss crystal grains arranged in the {110}<001> orientation exist in a cold-rolled sheet. In a two-time or more rolling method, orientations having a low degree of integration also exist in the cold-rolled sheet. Therefore, the orientations having the low degree of integration are also secondarily recrystallized at the time of final annealing, such that a magnetic flux density and core loss characteristics are deteriorated. Therefore, it is preferable that the cold rolling is once steel cold rolling and has a cold rolling ratio of 87% or more. In detail, the cold rolling ratio may be 87% to 90%, 87% to 91%, 87% to 92%, 87% to 93%, 87% to 94%, 87% to 95%, 87% to 96%, 87% to 97%, 87% to 98%, or 87% to 99%.

Decarbonization, recrystallization of the deformed structure, and nitriding using an ammonia gas are performed on the cold-rolled plate as described above. The decarbonization and nitriding process may be any one of a method of performing the nitriding using the ammonia gas after the decarbonization and the recrystallization end and a method of simultaneously using the ammonia gas so that the nitriding may be performed simultaneously with the decarbonization, which does not cause a problem in exerting an effect of the present invention.

It is preferable that an annealing temperature of the steel sheet in performing the decarbonization, the recrystallization, and the nitriding is in a range of 800 to 950° C. When the annealing temperature of the steel sheet is lower than 800° C., it takes a lot of time to perform the decarbonization. When the annealing temperature of the steel sheet is higher than 950° C., recrystallized grains are coarsely grown, such that crystal growth driving force is decreased. Therefore, stable secondary recrystallization is not formed. In addition, the annealing time is not a serious problem in exerting the effect of the present invention, but it is preferable that the annealing time is within 5 minutes in consideration of productivity.

Immediately before or after the decarburization nitriding annealing heat treatment ends, a portion or the entirety of an oxide layer present in an outer oxide layer formed on a surface of the steel sheet decarbonization-nitriding-annealed may be reduced and removed under a reducing atmosphere. Then, an annealing separating agent based on MgO may be applied to the steel sheet, and final annealing may be performed on the steel sheet for a long time to generate the secondary recrystallization, thereby forming a {110}<001> texture in which a {110} surface of the steel sheet is in parallel with a rolled surface and a <001> direction is in parallel with a rolling direction. Therefore, the grain-oriented electrical steel sheet having excellent magnetic characteristics may be manufactured.

Main objects of the final annealing are to form the {110}<001> texture by the secondary recrystallization, give an insulation property by forming a glass film by an reaction between the oxide layer formed at the time of decarbonization and MgO, and remove impurities damaging the magnetic characteristics. It is preferable that a final annealing temperature is a decarbonization annealing temperature or more to 1250° C. or less. In detail, the final annealing temperature may be 800° C. to 1250° C., 850° C. to 1250° C., or 900° C. to 1250° C. As a final annealing method, an atmosphere including one or more of nitrogen and hydrogen may be maintained in a temperature increase section before the secondary recrystallization is generated.

Therefore, a nitride, which is a grain growth inhibitor, may be protected to allow the secondary recrystallization to be grown well. After the secondary recrystallization is completed, the steel sheet is maintained for a long time under a 100% hydrogen atmosphere.

MODE FOR INVENTION

Hereinafter, Examples of the present invention and Comparative Examples are described. However, the following Examples are only exemplary embodiments of the present disclosure, and the present invention is not limited to the following Examples.

Example 1

A grain-oriented electrical steel sheet containing 0.004% of S, 0.0042% of N, 0.028% of Sol-Al, 0.028% of Sb, 0.07% of Sn, 0.028% of P, and 0.03% of C of which a content is changed depending on contents of Si and Mn as illustrated in Table 1, and containing balance Fe and other inevitable impurities as the remaining components was dissolved in a vacuum state, and ingot was produced, was heated to a temperature of 1150° C., and was then hot-rolled at a thickness of 2.3 mm. A hot-rolled plate was heated to a temperature of 1085° C., was maintained at 920° C. for 160 seconds, and was then quenched in water. The hot-rolled annealed sheet was pickled and was then rolled once to a thickness of 0.23 mm, and the cold-rolled sheet was maintained under a mixed gas atmosphere of humid hydrogen, nitrogen, and ammonia at a temperature of 860° C. for 200 seconds to perform a simultaneous decarbonization annealing heat treatment so that a content of nitrogen is 170 ppm.

MgO, which is an annealing separating agent, was applied to the steel sheet to finally anneal the steel sheet, the final annealing was performed under a mixed atmosphere of 25% nitrogen+75% hydrogen as a volume ratio at a temperature up to 1200° C., and after a temperature of the steel sheet arrives at 1200° C., the steel sheet was maintained for 10 or more hours under a 100% hydrogen atmosphere and was then furnace-cooled. Measurement values of magnetic characteristics in the respective conditions are illustrated in Table 1.

TABLE 1 Core Loss Magnetic Flux Si Mn C (W17/50, density (wt %) (wt %) (wt %) W/kg) (B10, Tesla) Division 2.56 0.07 0.015 1.3 1.86 Comparative Example 1 2.55 0.11 0.055 1.2 1.85 Comparative Example 2 2.55 0.09 0.042 0.84 1.94 Inventive Example 1 2.88 0.06 0.024 1.24 1.84 Comparative Example 3 2.89 0.11 0.062 1.3 1.86 Comparative Example 4 2.88 0.09 0.045 0.85 1.94 Inventive Example 2 3.02 0.07 0.025 1.25 1.86 Comparative Example 5 3 0.09 0.062 1.21 1.86 Comparative Example 6 3.03 0.09 0.049 0.82 1.91 Inventive Example 3 3.15 0.07 0.026 1.29 1.84 Comparative Example 7 3.14 0.1 0.065 1.26 1.84 Comparative Example 8 3.15 0.09 0.052 0.83 1.93 Inventive Example 4 3.35 0.06 0.029 1.23 1.82 Comparative Example 9 3.35 0.09 0.072 1.28 1.83 Comparative Example 10 3.34 0.06 0.058 0.79 1.91 Inventive Example 5 3.55 0.07 0.03 1.29 1.84 Comparative Example 11 3.57 0.09 0.075 1.25 1.84 Comparative Example 12 3.56 0.09 0.064 0.78 1.9 Inventive Example 6 3.84 0.07 0.029 1.27 1.83 Comparative Example 13 3.82 0.1 0.084 1.2 1.84 Comparative Example 14 3.79 0.09 0.066 0.8 1.9 Inventive Example 7 4.01 0.06 0.025 1.29 1.83 Comparative Example 15 4 0.1 0.085 1.25 1.84 Comparative Example 16 4.02 0.09 0.072 0.77 1.9 Inventive Example 8

It may be seen from the above Table 1 that magnetic characteristics are significantly improved in Inventive Examples in which −0.32×Mn(wt %)+0.012×Si(wt %)+0.016≤C(wt %)≤−0.014×Mn(wt %)+0.02×Si(wt %) where a content of C is a content relationship among Si, Mn, and C as compared with Comparative Examples.

Example 2

A grain-oriented electrical steel sheet containing 3.35% of Si, 0.061% of C, 0.058% of Mn, 0.004% of S, 0.004% of N, 0.029% of Sol-Al, 0.032% of P, and Sn, Sb, and Cr of which contents are changed as illustrated in Table 2, and containing balance Fe and other inevitable impurities as the remaining components was dissolved in a vacuum state, and ingot was produced, was heated to a temperature of 1140° C., and was then hot-rolled at a thickness of 2.3 mm. A hot-rolled plate was heated to a temperature of 1080° C., was maintained at 915° C. for 162 seconds, and was then quenched in water. The hot-rolled annealed sheet was pickled and was then rolled once to a thickness of 0.23 mm, and the cold-rolled sheet was maintained under a mixed gas atmosphere of humid hydrogen, nitrogen, and ammonia at a temperature of 850° C. for 200 seconds to perform a simultaneous decarbonization annealing heat treatment so that a content of nitrogen is 180 ppm.

MgO, which is an annealing separating agent, was applied to the steel sheet to finally anneal the steel sheet, the final annealing was performed under a mixed atmosphere of 25% nitrogen+75% hydrogen as a volume ratio at a temperature up to 1200° C., and after a temperature of the steel sheet arrives at 1200° C., the steel sheet was maintained for 10 or more hours under a 100% hydrogen atmosphere and was then furnace-cooled. Measurement values of magnetic characteristics in the respective conditions are illustrated in Table 2.

TABLE 2 Core Loss Magnetic Flux Sn Sb Cr (W17/50, Density (wt %) (wt %) (wt %) W/kg) (B10, Tesla) Division 0.03 0.01 — 0.86 1.91 Comparative Example17 0.03 0.01 0.04 0.80 1.93 Inventive Example9 0.03 0.05 0.01 0.83 1.92 Comparative Example18 0.03 0.05 0.04 0.78 1.93 Inventive Example10 0.05 0.01 0.01 0.85 1.92 Comparative Example19 0.05 0.01 0.04 0.78 1.93 Inventive Example11 0.05 0.05 0.01 0.84 1.91 Comparative Example20 0.05 0.05 0.04 0.79 1.93 Inventive Example12 0.07 0.01 0.01 0.84 1.91 Comparative Example21 0.07 0.01 0.04 0.79 1.94 Inventive Example13 0.07 0.05 0.02 0.84 1.92 Comparative Example22 0.07 0.05 0.05 0.79 1.93 Inventive Example14

It may be seen from the above Table 2 that magnetic characteristics are improved in Inventive Examples in which Sn(wt %)+Sb(wt %)≤5×Cr(wt %) where a content of Cr is a content relationship among Sn, Sb, and Cr as compared with Comparative Examples.

While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A grain-oriented electrical steel sheet containing 2.0 wt % or more to 5.0 wt % or less of Si, 0.005 wt % or more to 0.04 wt % or less of acid-soluble Al, 0.01 wt % or more to 0.2 wt % or less of Mn, 0.01 wt % or less (excluding 0 wt %) of N, 0.01 wt % or less (excluding 0 wt %) of S, 0.01 wt % or more to 0.05 wt % or less of Sb, 0.02 wt % or more to 0.08 wt % or less of C, 0.0005 wt % or more to 0.045 wt % or less of P, 0.03 wt % or more to less than 0.08 wt % of Sn, and 0.01 wt % or more to 0.2 wt % or less of Cr, and containing balance Fe and other inevitable impurities.
 2. The grain-oriented electrical steel sheet of claim 1, wherein: the grain-oriented electrical steel sheet satisfies the following Equation 1 calculated by contents (wt %) of the respective components: −0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si.  [Equation 1]
 3. The grain-oriented electrical steel sheet of claim 1, wherein: the grain-oriented electrical steel sheet satisfies the following Equation 2 calculated by contents (wt %) of the respective components: Sn+Sb≤5Cr.  [Equation 2]
 4. The grain-oriented electrical steel sheet of claim 1, wherein: the grain-oriented electrical steel sheet satisfies the following Equation 1 and the following Equation 2 calculated by contents (wt %) of the respective components: −0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si, and  [

1] Sn+Sb≤5Cr.  [

2]
 5. The grain-oriented electrical steel sheet of claim 4, wherein: a fracture of an austenite phase in the grain-oriented electrical steel is 20 to 30%.
 6. The grain-oriented electrical steel sheet of claim 5, wherein: an area of crystal grains of which a ratio between the longest diameters and the shortest diameters is 1.0 or more among crystal grains of which lengths of the shortest diameters are 3 mm or more is 5% or more of an area of all the crystal grains.
 7. A method for manufacturing a grain-oriented electrical steel sheet, comprising: reheating a steel slab containing 2.0 wt % or more to 5.0 wt % or less of Si, 0.005 wt % or more to 0.04 wt % or less of acid-soluble Al, 0.01 wt % or more to 0.2 wt % or less of Mn, 0.01 wt % or less (excluding 0 wt %) of N, 0.01 wt % or less (excluding 0 wt %) of S, 0.01 wt % or more to 0.05 wt % or less of Sb, 0.02 wt % or more to 0.08 wt % or less of C, 0.0005 wt % or more to 0.045 wt % or less of P, 0.03 wt % or more to less than 0.08 wt % of Sn, and 0.01 wt % or more to 0.2 wt % or less of Cr, and including balance Fe and other inevitable impurities; manufacturing a steel sheet by performing hot rolling, hot-rolled sheet annealing, and cold rolling on the reheated steel slab; performing decarbonization annealing and nitriding annealing on the cold-rolled steel sheet; and finally annealing the decarbonization annealed and nitriding annealed steel sheet.
 8. The method for manufacturing a grain-oriented electrical steel sheet of claim 7, wherein: the steel slab satisfies the following Equation 1 calculated by contents (wt %) of the respective components: −0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si.  [Equation 1]
 9. The method for manufacturing a grain-oriented electrical steel sheet of claim 7, wherein: the steel slab satisfies the following Equation 2 calculated by contents (wt %) of the respective components: Sn+Sb≤5Cr.  [Equation 2]
 10. The method for manufacturing a grain-oriented electrical steel sheet of claim 7, wherein: the steel slab satisfies the following Equation 1 and the following Equation 2 calculated by contents (wt %) of the respective components: −0.32Mn+0.012Si+0.016≤C≤−0.014Mn+0.02Si, and  [Equation 1] Sn+Sb≤5Cr.  [Equation 2]
 11. The method for manufacturing a grain-oriented electrical steel sheet of claim 10, wherein: in the reheating of the steel slab, a temperature is 1000 to 1250° C.
 12. The method for manufacturing a grain-oriented electrical steel sheet of claim 11, wherein: in the manufacturing of the steel sheet by performing the hot rolling, the hot-rolled sheet annealing, and the cold rolling on the reheated steel slab, a hot-rolled sheet annealing temperature is 900 to 1200° C.
 13. The method for manufacturing a grain-oriented electrical steel sheet of claim 12, wherein: in the manufacturing of the steel sheet by performing the hot rolling, the hot-rolled sheet annealing, and the cold rolling on the reheated steel slab, a cold rolling thickness is 0.10 mm or more to 0.50 mm or less.
 14. The method for manufacturing a grain-oriented electrical steel sheet of claim 13, wherein: in the manufacturing of the steel sheet by performing the hot rolling, the hot-rolled sheet annealing, and the cold rolling on the reheated steel slab, the cold rolling is performed as once cold rolling of which a cold rolling ratio is 87% or more.
 15. The method for manufacturing a grain-oriented electrical steel sheet of claim 14, wherein: in the performing of the decarbonization annealing and the nitriding annealing on the cold-rolled steel sheet, the decarbonization annealing and the nitriding annealing are simultaneously performed, the nitriding annealing is independently performed after the decarbonization annealing, or the decarbonization annealing is independently performed after the nitriding annealing.
 16. The method for manufacturing a grain-oriented electrical steel sheet of claim 15, wherein: in the performing of the decarbonization annealing and the nitriding annealing on the cold-rolled steel sheet, the decarbonization annealing and the nitriding annealing are simultaneously performed, and an annealing temperature is 800 to 950° C.
 17. The method for manufacturing a grain-oriented electrical steel sheet of claim 16, further comprising: before the final annealing of the decarbonization annealed and nitriding annealed steel sheet, applying an annealing separating agent to the decarbonization annealed and nitriding annealed steel sheet.
 18. The method for manufacturing a grain-oriented electrical steel sheet of claim 17, wherein: in the final annealing of the decarbonization annealed and nitriding annealed steel sheet, a final annealing temperature is 800 to 1250° C.
 19. The method for manufacturing a grain-oriented electrical steel sheet of claim 18, wherein: the final annealing of the decarbonization annealed and nitriding annealed steel sheet is performed under an atmosphere including one or more of nitrogen and hydrogen, and is performed under a 100% hydrogen atmosphere after a temperature arrives at the final annealing temperature. 